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A history of ground-based rodent eradication techniques 
developed in New Zealand, 1959-1993 

B. W. Thomas 1 and R. H. Taylor 2 
1 Landcare Research, Private Bag 6, Nelson, New Zealand. E-mail: 
2 RH Taylor Associates, 22 Waterhouse St, Nelson, New Zealand. 

Abstract Eradicating rats from islands was for decades deemed highly desirable but considered practically impos- 
sible. This paper documents the development of ground-based rodent eradication techniques using bait stations in New 
Zealand up until 1993. The work culminated in a successful operation to eradicate Norway rats (Rattus norvegicus) 
from 3100 ha Langara Island in the Queen Charlotte Islands, Canada, in 1995. 

Keywords Eradication; rat, Rattus; islands; bait station; rolling front. 


Introductions of rodents to new regions during centuries 
of exploration and colonisation around the globe are 
recognised worldwide as a major conservation problem 
(Atkinson 1985). New Zealand ecosystems developed 
without terrestrial mammalian predators, the only land 
mammals being native bats. The flora and fauna that 
evolved through long oceanic isolation during the Tertiary 
were vulnerable to the depredation of introduced mam- 
mals, and many of the extinctions that have occurred in 
this country can be attributed to the introduction of ro- 
dents (King 1984). 

The first of the four rodent species introduced into New 
Zealand was the Pacific rat {Rattus exulans), which ar- 
rived with Maori, perhaps up to 2000 years ago (Holdaway 
1996). This rat was transported widely around the Pacific 
by Polynesian peoples (Wodzicki and Taylor 1984) who 
utilised it as food. There were likely to have been inten- 
tional liberations that, along with natural dispersal, resulted 
in the species becoming widespread on the main New 
Zealand islands, as well as establishing on many offshore 
islands (Atkinson and Towns 2001). 

One of the first documented records of Eurasian rodents 
in New Zealand is the account given by Anders Spaarman, 
a naturalist with Captain Cook, who described rats (prob- 
ably Norway rats - Rattus norvegicus) coming ashore at 
Pickersgill Harbour when the Resolution was beached for 
careening in Dusky Sound, Fiordland, in 1773 (Rutter 
1953). During the 19 th century, the ship rat (Rattus rattus) 
and house mouse (Mus musculus) also became established 
in New Zealand (Atkinson 1973; Taylor 1 975 , 1 978), prob- 
ably accidentally as ship visits to the new colony of New 
Zealand increased in the mid- 19 th century. 

The introduction of rodents has had a significant impact 
on native animal and plant species. What was accepted as 
unavoidable by colonisers of the day has been rued by 
naturalists, scientists, and conservationists ever since. In 
the 25 years since serious consideration was first given to 
the possibilities of rectifying this major ecological prob- 
lem, eradication of rodents from islands has become an 

accepted conservation management tool - now used with 
much success in various parts of the world. 

We describe here the history and development of ground- 
based, rodent eradication operations using bait stations in 
New Zealand, which led to the successful campaign to 
eliminate Norway rats from Langara Island in the Queen 
Charlotte Islands (Haida Gwaii) in Canada - at over 3 100 
hectares, the largest rat eradication achieved to date. 


With the establishment in New Zealand of various Gov- 
ernment wildlife and science agencies in the mid- 1900s, a 
better understanding of the distribution of rodents in New 
Zealand began to emerge (Wodzicki 1950; Watson 1956, 
1961), together with a greater appreciation of the detri- 
mental effect these predators were having in our ecosys- 
tems. A graphic example was the devastation and extinc- 
tions caused by ship rats on Big South Cape Island 
(Taukihepa), in the early 1960s where several locally en- 
demic birds were extirpated (Bell 1978). Removal of ro- 
dents seemed insurmountable and understandably efforts 
were focussed on ongoing methods of control, rather than 

For example, depredation of white-faced storm petrels 
(Pelagodroma marina) by Norway rats was noted in 1959 
on Maria Island (1 ha) in the inner Hauraki Gulf (Fig 1). 
During the 1959 to 1961 breeding seasons, Mr A 
McDonald and other members of the Forest and Bird Pro- 
tection Society, assisted in part by Don Merton and a £5.00 
grant from Wildlife Service for poison, endeavoured to 
protect white-faced storm petrels on Maria Island and the 
adjacent David Rocks (less than 1 ha). This attempt used 
the warfarin-based rodenticide 'Rid-rat', which was dis- 
tributed around petrel colonies (Merton 1961, 1962). From 
later visits in the mid-60s it appeared that rats had almost 
certainly been eradicated from each of these small islands 
(Moors 1985). 

During the early 1970s, research on burrowing seabirds 
on Whale Island (Moutohora) in the Bay of Plenty included 
studies of the effects of Norway rats on grey-faced petrels 

Pages 301-310 In Veitch, C. R. and Clout, M. N. (eds.). Turning the tide: the eradication of invasive species. IUCN SSC Invasive 
Species Specialist Group. IUCN, Gland, Switzerland and Cambridge, UK. 

Turning the tide: the eradication of invasive species 

(Pterodroma macroptera gouldi) and sooty shearwaters 
(Puffinus griseus) (Imber 1978). A rat control programme 
was carried out over a small, 6.3 ha area of low-density 
seabird burrowing on the island, in which 800 (4 oz) pack- 
ets of the warfarin-based poison "Prodide" were distrib- 
uted, reducing rats to low numbers. However, re-invasion 
from outside the study area soon began to occur 
(Bettesworth 1972; Imber 1978). 

Similarly, Norway rats were believed to be having a detri- 
mental effect on breeding sooty shearwaters and flesh 
footed shearwaters (Puffinus carneipes) on Titi Island 
(32 ha) in the Marlborough Sounds. In the belief that total 
eradication was unachievable, Brian Bell (1969) suggested 
to the Lands and Survey Department that a control 
programme be put in place to reduce rat numbers during 
the chick fledging period. In December 1970, Dick Veitch 
of the Department of Internal Affairs, Wildlife Branch, 
laid 3 10 4oz packets of "Prodide" at about 15 m intervals 
along or near the main ridge of the island. Subsequent 
checks indicated that the rat population had been signifi- 
cantly reduced (Veitch 1970, 1971). Follow-up was inter- 
mittent and rats continued to be recorded. There was fur- 
ther poisoning around the colonies in December 1973, but 
checks by Lands and Survey personnel in March 1975 in- 
dicated rats were still present. Another application of poi- 
son was undertaken in May 1975, but there was no fol- 
low-up monitoring until we visited in 1981-1982. Our in- 
tention was to use Titi for an experimental eradication of 
Norway rats, but after continuous trapping over a six month 

period (approximately 9000 fenn trap-nights) we found 
rats were no longer present (Gaze 1983). 

Despite widespread concern at the possibility of rats reach- 
ing rodent-free islands, it was not until November 1976 
that a concerted effort was made to address the problem. 
On advice from Ian Atkinson, Rowley Taylor and Brian 
Bell, members of the Outlying Islands Committee, and at 
the urging of John Yaldwyn (National Museum of New 
Zealand), a symposium on the 'Ecology and Control of 
Rodents in New Zealand Nature Reserves' was organised 
by the Department of Lands and Survey (Coad 1978). This 
conference brought together over 50 of New Zealand's 
rodent researchers, wildlife practitioners and managers of 
island reserves - representing many government depart- 
ments, research organisations, museums, and universities. 

Presented papers and discussion ranged widely from the 
effects of rodents on ecosystems to the possibilities for 
control and eradication. It is indicative of general thinking 
of the day that despite a report of the apparent eradication 
of rats from Maria Island and the David Rocks (we did 
not know the outcome for Titi Island at this stage), in a 
final comment the Chairman, John Yaldwyn, concluded: 
"We have control methods, and methods for reducing popu- 
lations, but complete extermination on islands is remote 
or at least a very very difficult thing indeed. "(Yaldwyn 
1978). Nevertheless, the meeting overall provided the 
impetus for several individuals to pursue their ideas to 
develop methods for eradication of rodents. 


Although most rodent research in New Zealand continued 
to be directed towards the distribution and ecology of rats 
and mice (Wildlife Research Liaison Group 1984), work 
now began on developing rat eradication techniques for 
islands. This was aided by the production of new and im- 
proved toxins in the form of second generation anticoagu- 
lants in proprietary rodenticides such as 'Talon' 
(brodifacoum) and 'Storm' (flocoumafen). 

Recognising the difference in approach needed between 
eradication and control operations, Phil Moors of the New 
Zealand Wildlife Service began to test the feasibility of 
eradicating Norway rats from islands by undertaking dif- 
fering poison trials on Motuhoropapa (9.5 ha) and Otata 
(21 .8 ha) islands in the Noises Group (Moors 1978, 1979). 
It was thought eradication had been inadvertently achieved 
on Motuhoropapa in 1977-1978 as a result of his prelimi- 
nary trapping study, and Moors postponed his planned 
poison programme to test this result. However, monitor- 
ing revealed that rats were still present in low numbers 
and the poison programme was reinstated in 1981. This 
involved a combination of compound 1080 (sodium 
monofluoroacetate)-impregnated grain, distributed in 75 
plastic bait stations placed at 50 m intervals along existing 
tracks, plus 1080 paste spread in likely haunts around the 
coast and on the offshore stacks. A few months later, the 
1080 baits were replaced with 0.005% brodifacoum 'Talon' 

— i 


Motuopao Island 

Hen and Chickens Group 
Lady Alice Island 
Coppermine Island 

Hauraki Gulf 
Noises Group 
David Rocks 
Motuhorapapa Island 
Otata Island 
Maria Island 

Double Island 
Korapuki Island 


Marlborough Sounds 
Titi Island 
Awaiti Island 
Tawhitinui Island 

Breaksea Sound 
Breaksea Island 
Hawaa Island 

Dusky Sound 

Stewart Island" 


Haulashore Island 

400 km 

- Paterson Inlet 
Ulva Island 

Big South Cape island 

Campbell Island 


Fig. 1 Island localities mentioned in text. 


Thomas and Taylor: Ground-based rodent eradication techniques 

WB 50 waxed baits and 0.0 1 % brodifacoum paste (Moors 
1985). Kill traps (over 1400 trap-nights) were also set. No 
rats or sign were found after February 1983. 

In 1979, bait stations were established on Otata Island 
(21 ha) on a grid at 40 m spacings. Poisoning was under- 
taken in two stages, the first using Compound 1080 in a 
mixture of rolled oats and fish-flavoured cat food, little of 
which was touched. Single 'Talon' WB 50 baits (about 
7500) were then placed at 10 x 5 m spacings over the is- 
land, but a second island-wide poisoning campaign was 
deemed necessary in September 1980 after rodent drop- 
pings were found (Moors 1985). Eradication was con- 
firmed on both these islands in 1987 (Veitch and Bell 
1990). Although this work eventually met with success, it 
required considerable time and effort and led Moors (1985) 
to recommend: "use as many methods of killing rats as 
you can, and never rely on one weapon alone". 

Stemming from this work, Ian McFadden (1984) devel- 
oped and tested bait stations, dispensing silos and various 
forms of baits and attractants on Pacific rats on Lady Alice 
Island. This technology was refined during eradication tri- 
als of Pacific rats in 1983-1984 on Rurima Island (6 ha) 
in the Bay of Plenty. After pre-feed trials, he used 1080- 
impregnated kibbled maize dispensed from 30 gravity-fed 
silos, but the rats did not take the bait (probably because 
of the taste of dyes used in manufacture). Subsequently, 
undyed kibbled maize impregnated with the anti-coagu- 
lant bromadiolone was used in an expanded array of silos 
(41 in total). Rodent feeding sign at the silos and monitor- 
ing using snap traps, indicated that eradication was prob- 
ably achieved within three months of laying the poison 
(McFadden and Towns 1991). 

This successful method was applied on Korapuki Island 
(18 ha), where McFadden improved his silo methodology 
and eradicated Pacific rats after just one application of 
bromadiolone-impregnated toxic kibbled maize 
(McFadden and Towns 1991). McFadden's next experi- 
ment on Pacific rats, on Double Island in 1989, compared 
the cost and effectiveness of bromadiolone-poisoned grain 
in silo bait stations on one half of the island against hand 
broadcast distribution of commercially-available 
flocoumafen-based "Storm" rodent pellets on the other half 
(McFadden 1992). Both techniques achieved successful 
eradication, but the cost of broadcasting baits was mar- 
ginally cheaper. Because of potential cost savings 
McFadden explored the development of aerial broadcast 
technology in which poison is distributed from spreader 
buckets slung under helicopters (McFadden and Green 
1994). It is an eradication technique now widely used in 
New Zealand that has proved especially effective in diffi- 
cult terrain and/or isolated situations, and with which suc- 
cess is being achieved on larger and larger islands 
(Cromarty et al. 2002). 

While Wildlife Service was undertaking their early work 
on northern islands, the Department of Scientific and In- 
dustrial Research (DSIR) Ecology Division, was engaged 
in a series of rodent distribution surveys — involving Bruce 

Thomas (BWT) and others — in the Nelson/Marlborough 
region under the leadership of Rowley Taylor (RHT). 
Through this work (Taylor 1984) links were developed 
with the Department of Lands and Survey, who in 1980— 
1981 were considering rat eradication trials on Campbell 
Island. RHT recommended that, given current knowledge, 
a more appropriate plan would be for Ecology Division to 
cooperate in trials of rat eradications on smaller, readily- 
accessible islands in the Marlborough Sounds. 

In planning the trials, we adopted a different approach from 
that recommended by Moors (1985). We tested a single- 
hit, single-poison methodology, taking account of the 
known behaviour of rats. Our aim was to develop a sys- 
tem of dispensing a proven rodenticide into the territory 
of every rat on an island, in a way that would minimise 
non-target poisoning, and monitor the effectiveness of the 
campaign as it progressed (Taylor and Thomas 1989). 

In a joint DSIR Ecology Division/Marlborough Sounds 
Maritime Park programme, David Taylor and two other 
Lincoln College students, under Rangers Dave Maizey and 
Bob Ryan, carried out the fieldwork for the first two trials. 
The initial experiment was against ship rats on Awaiti Is- 
land (2 ha). Simple bait stations with a top-loading access 
slot and a clip-in cover were made from 65 mm diameter 
plastic drainage pipe ('Novacoil'). About 120 bait stations 
were sited approximately 15-20 m apart over the island 
and a single 15 g "Talon WB 50" pellet containing 0.005% 
brodifacoum, a second generation anticoagulant rat poi- 
son, was placed in each tunnel. Pellets were replaced as 
necessary during weekly checks (i.e. up to five times be- 
tween 10 March 1982 and 16 April 1982) - from which 
time no further baits were taken. Follow-up monitoring 
(kill traps and tracking tunnels) confirmed eradication had 
been achieved (D. Taylor 1983). 

The second trial also targeted ship rats on the adjacent, 
forested, Tawhitinui Island (21 ha). A network of tracks 
was cut to give access to the coast at regular intervals from 
along the main ridges of the island. A total of 374 bait 
stations of 65 mm plastic drainage pipe were placed along 
these tracks, achieving a variable grid of 25-50 m. A single 
"Talon WB 50" bait was placed in each of the tunnels and 
checked and replenished weekly from 26 January 1983 
until the poison-take stopped on 15 February 1983. Poi- 
son baits were left in place until February 1984 and the 
campaign was considered a success in August 1984 after 
follow-up monitoring (baited tracking tunnels and snap- 
trap lines) detected no further sign of rats (D. Taylor 1984). 

About the same time, a review of Wildlife Service research 
priorities gave the highest priority rating for new predator 
projects to "The development of eradication methods for 
use on small islands" (Crawley 1983). We were already 
convinced of the potential to further develop bait station 
technology as a rat eradication technique for much larger 
islands, but a general scepticism of this methodology per- 
sisted amongst administrators, researchers and wildlife 
practitioners. For example, in a priority listing of 1 1 re- 


Turning the tide: the eradication of invasive species 

search topics on rodents, the Wildlife Research Liaison 
Group (1984) gave top priority to mapping rodent distri- 
bution. By comparison, 'methods of eradication or con- 
trol' were placed low on the list at priority 9, with specific 
reference made only to possible biological methods. De- 
spite our success in the Marlborough Sounds, the use of 
rodenticides in bait stations did not rate a mention. In 1983, 
influenced by the problems that Moors was encountering 
on the Noises Islands, Ian Atkinson voiced the then-com- 
monly-held view that "Once rats have established on an 
island, it is generally not feasible to remove them unless 
the island is very small. " (Atkinson 1986). 

The Department of Lands and Survey was keen to con- 
tinue its support for research aimed at rat eradication on 
Campbell Island, and in 1983 DSIR Ecology Division 
negotiated a research contract to further this objective. 
Graeme Taylor was employed to study Norway rats on the 
island to assist planning for an eventual eradication cam- 
paign. Key aspects of his research were rat distribution, 
density, breeding, food, habits, and home range size (Tay- 
lor 1986). 


Norway rats were confirmed present on Breaksea Island 
(170 ha) and the then unnamed Hawea Island (9 ha) dur- 
ing an ecological survey of islands in Breaksea Sound in 
1974 (Thomas 1975). The possibility of eradicating Nor- 
way rats from Breaksea Island to create a predator-free 
environment in which to translocate the last few Fiordland 
kakapo (Strigops habroptilus) was discussed in depth by 
the team members during the survey. Enthused by the 'Ro- 
dents in Reserves' symposium, and encouraged by the suc- 
cess of the Marlborough Sounds trials, during subsequent 
work for the Fiordland National Park Board, BWT for- 
mulated ideas to undertake further development of bait 
station rodent eradication technology in Breaksea Sound 
(Taylor etal. 1986). 

In April 1984 BWT, RHT and Fiordland National Park 
staff were joined by the director of Ecology Division on 
another ecological survey of islands in Breaksea and Dusky 
Sounds (Thomas and Taylor 1988). Our director was less 
than convinced with our proposal to eradicate rats from 
Hawea and Breaksea Islands, believing that eradication of 
rats from an island as large and rugged as Breaksea was 
not achievable - a sentiment echoed time and again from 
many quarters. Undaunted, and with the support of De- 
partment of Lands and Survey colleagues, we gained a 
small grant and an offer of logistical support from the 
Fiordland National Park Board. This enabled us to finalise 
plans to undertake an experimental eradication operation 
against Norway rats on Hawea Island, with the clear in- 
tention of expanding the programme to include Breaksea 
Island should we be successful. 

We believed that the single best method available should 
be employed to achieve eradication in the shortest pos- 

sible time. In undertaking the Hawea Island campaign we 
hoped to develop existing technology further to overcome 
problems such as bait station design, neophobia, bait avoid- 
ance and poison resistance, and monitoring success - all 
of which had compromised previous eradication opera- 
tions to various degrees. With the help of Graeme Taylor 
and the voluntary assistance of several other people (Tay- 
lor and Thomas 1986), a track system and a preliminary 
programme to monitor ecological changes following rat 
eradication was completed on Hawea Island in 1986. Sev- 
enty-three 100 mm diameter plastic drainage pipe bait sta- 
tions, each 400 mm long, were placed on a 40 m grid over 
the island three weeks before poisoning, to minimise 
neophobic avoidance by rats. From 1 1-22 April 1986, two 
"Talon WB 50" baits were placed in each tunnel and 
checked and replenished daily - a monitoring regime that 
enabled collection of precise data on rat activity. Eradica- 
tion was accomplished in less than two weeks, and the 
system was self-monitoring and required no special effort 
to get the last rat (Taylor and Thomas 1989). 

We could now recommend with confidence that a similar 
poison campaign be carried out on 170 ha Breaksea (Tho- 
mas and Taylor 1988). A project proposal submitted to 
DSIR gained research support for a Breaksea Island cam- 
paign for the period 1987-1989 (Ecology Division 1987). 
However, for the programme to go ahead it was essential 
that our draft work plan (Taylor and Thomas 1987) be 
accepted by the newly-formed Department of Conserva- 
tion (DOC). The plan required an eight bunk hut and two 
bivvies to be built, hundreds of person-hours cutting tracks, 
the production of up to 1000 plastic drainage pipe bait 
stations, and 500 kg of "Talon 50 WB" rat poison - over 
NZ$50,000 for materials alone. The new managers we were 
dealing with were reluctant to commit resources, uncon- 
vinced that we could achieve eradication on such a large 
scale. By then, based on McFadden's work, conventional 
thinking was that "rodent extermination on islands up to 
40 or 50 ha might be possible. " (Towns 1988). 

Two Te Anau DOC staff, Tom Paterson and Ron Peacock, 
shared our vision and were instrumental in the project be- 
ing designated an official "Fiordland National Park Cen- 
tennial Year Project". They secured some old Ministry of 
Works buildings, which provided the materials for the huts, 
successfully negotiated with ICI (Imperial Chemical In- 
dustries, now Zeneca) to donate the poison, and organised 
for the participation of "Operation Raleigh". A commit- 
ment by DOC was made to go ahead with the project, and 
Ian Thorne took responsibility for coordinating prepara- 
tion of the island (Department of Conservation 1988). In 
1987, several teams of young people from New Zealand 
and around the world, paying for an outdoor adventure 
experience with "Operation Raleigh", spent weeks under 
canvas in harsh conditions on Breaksea Island marking 
routes and cutting tracks, which were completed by DOC 
staff and voluntary helpers. 

The Breaksea Island campaign was similar to the Hawea 
Island poison operation, but stations were more widely 
spaced (50 m apart) along contour tracks cut at 60 m ver- 


Thomas and Taylor: Ground-based rodent eradication techniques 

tical intervals from the coast to the summit. Thus, the sur- 
face distance between lines varied from about 30-100 m 
depending on the steepness of the terrain. Extra stations 
were installed at 25 m intervals along the main access 
ridges, with all 743 bait stations in place two months be- 
fore poisoning. Six large weather-proof stations, each con- 
taining 50 "Talon WB 50" baits, were positioned by heli- 
copter on inaccessible cliffs and offshore stacks. During 
the main poisoning operation (26 May to 16 June 1988), 
stations were loaded with two "Talon WB 50" baits and 
checked and replenished daily. Six poison operators, led 
by Ian Thorne, each had responsibility for a section of 
island. Bait-take was analysed daily to monitor the chang- 
ing status of the rat population. As the operation progressed 
as predicted, even the sceptics in the team changed their 
views on our chance of success. On day 21, leaving the 
island loaded with four talon baits per station, we were 
confident that only two already-poisoned rats remained 
alive. Two years of post-poison monitoring confirmed our 
success (Taylor and Thomas 1993). 

This created the largest predator-free island in Fiordland 
and advanced understanding of eradication technology. 
However, the most important outcome was improved con- 
fidence amongst administrators, conservation practitioners, 
politicians and the public alike, that eradication of rodents 
was achievable on a large scale - that money was not be- 
ing squandered in attempting such operations. This was 
aided by raising awareness of the project through various 
media, the most important being the production of the 
Television New Zealand Wild South documentary 'Battle 
for Breaksea Island' (Natural History New Zealand Ltd 
1990). This 26 minute television documentary, shown in 
New Zealand and overseas, has had a tremendous impact 
on predator eradication efforts. 

1989 TO 1993: THE "ROLLING FRONT" 

The Breaksea Sound work was to us a preliminary step 
towards eradicating rats from Kapiti Island (Thomas and 
Taylor 1988), a 1970 ha island sanctuary of world renown 
and of particular importance to the conservation of sev- 
eral critically endangered bird species (Maclean 1999). 
Removal of feral stock had been achieved, and possums 
eradicated in 1986 (Cowan 1992; Maclean 1999). Pacific 
and Norway rats were the only introduced mammalian 
predators that remained and we felt the time was right to 
give consideration to their eradication. We prepared a dis- 
cussion document proposing that because of the island's 
size a ground-based poison campaign be undertaken se- 
quentially in three stages in what we termed a "rolling front" 
regime, and recommended that it first be tested elsewhere 
(Thomas and Taylor 1991). For reasons of size, shape, 
ease of access and the fact that it had Norway rats, we 
suggested that Ulva Island (270 ha) in Paterson Inlet, 
Stewart Island, would be the best place to undertake such 
trials. Southland Conservancy of DOC agreed that their 
existing programme for Ulva, based on methods used on 
Breaksea Island, should be modified and the island used 

to trial the rolling front on a "research by management" 

Responsibility for this project had been given to DOC of- 
ficer Lindsay Chatterton who implemented the changes 
necessary to undertake the more complex "rolling front" 
programme. The island was divided into three blocks of 
70-100 ha, which were to be poisoned sequentially. A to- 
tal of 282 bait stations were placed along 47 lines to achieve 
a 100x100 m grid over the whole island. There were con- 
cerns about the possibility of unnecessary amounts of poi- 
son entering the food chain, and the effects of this on non- 
target species and the environment. To study optimum 
baiting levels, each block received a different loading of 
"Talon WB 50" poison, from an extremely low dosage in 
Block 1 to a dose somewhat less than we used on Breaksea 
Island in Block 3 (Fig. 2); and the check-replenishment 
regime was pulsed according to the particular stage of the 
campaign. Just before the poisoning began we learned that 
"Talon WB 50" baits had been in regular and widespread 
use on Ulva for rat control for over 10 years. This pre- 
sented a possibility that the programme could be compro- 
mised by bait avoidance or poison resistance in the rat 
population (Taylor and Thomas 1989). 

Poisoning began in Block 1 on 6 July 1992, with two Talon 
baits per station checked and replenished by two opera- 
tors every two days. Three weeks later, poisoning started 
in Block 2 with four baits per station. A further four weeks 
later, in late August, eight baits per station were applied in 
Block 3. As the "rolling front" moved ahead into the next 
block, bait loadings were doubled and the frequency of 
checks reduced. On 24 October, all stations on Ulva Is- 
land were loaded with 10 baits and less frequent, but regu- 
lar, checks continued until April 1993. Non-toxic indica- 
tor baits were also distributed over the island at this time 
and fenn traps set to monitor success and catch surviving 
rats. There were marked differences in results between 
blocks. Maximum daily bait-take peaked much earlier and 
declined more rapidly in high-dosage Block 3 compared 
with low-dosage Block 1, where the peak was delayed, 
the high bait-take period was protracted and the overall 
period of bait-take continued for longest. 



f0 150 

- I— ' 

1 100 

I 50 




♦ Langara 

♦ Ulva-3 
. Ulva-2 


20 40 60 80 100 120 

Days to 99% bait-take 

Fig. 2 Relationship between bait availability 
and duration of rat-eradication campaigns. 


Turning the tide: the eradication of invasive species 

Our concerns at the possibilities of bait avoidance were 
also realised. A number of poison-resistant or bait-shy in- 
dividuals survived on Ulva, after the rest of the rats had 
been poisoned. At least two females became pregnant sev- 
eral months after Talon bait was laid in their block, and 
one gave birth to at least three young before being finally 
trapped. Other trapped rats were found to have 
brodifacoum concentrations in their livers well in excess 
of that usually found in rats poisoned by Talon. The 
brodifacoum-resistant rats were detected through the moni- 
toring regime, which included the use of non-toxic indica- 
tor baits as well as the self-monitoring nature of the bait- 
station method. Hotspots were eventually cleared by trap- 
ping (11 rats caught) and/or bromadiolone-poisoned 
crushed maize. 

In response to concerns by some DOC staff that 100 m 
spacings of bait stations would be too wide, and to record 
responses of rats living adjacent to the poison fronts, 18 
Norway rats were live-trapped in the vicinity of the poi- 
son fronts shortly before or during poisoning. Each rat was 
fitted with a radio transmitter, released and tracked. Of 1 1 
transmitters used in the study, two failed after six and 18 
days respectively and were never recovered, a male rat 
lost its transmitter after two days (probably through poor 
fitting), a female rat in poor condition was located dead 
the day after she was caught and, where appropriate, some 
transmitters recovered from poisoned rats were reused. Six 
of the transmitters had their aerials chewed off at the base 
but emitted strong enough signals for tracking to continue. 

Rats were tracked for periods ranging from two to 69 days. 
Dens of marked rats were located daily and as many local- 
ity fixes as possible were obtained for each rat during their 
period of night-time activity. Single, linear movements of 
over 600 m in a night were recorded and all telemetered 
rats were recorded in the vicinity of one or more bait sta- 
tions. As poisoning progressed and population pressure 
reduced, some rats moved up to 400 m into poison-acti- 
vated areas from adjacent non-poison blocks. Most of the 
telemetered individuals died less than two weeks after 
Talon had been laid in their block, however, three of the 
study animals persisted for more than a month after bait- 
ing commenced. This pattern was quite different to what 
we had experienced in the Hawea and Breaksea campaigns, 
and a further indication that a percentage of the rat popu- 
lation on Ulva Island was either bait shy or poison resis- 

Concurrent with eradication operations on Hawea, 
Breaksea and Ulva Islands, important developmental work 
was also being undertaken in other areas by DOC work- 
ers. In 1989, Paul Jansen adapted the Breaksea Island 
work plan to eradicate Norway rats from Mokoia Island 
(135ha)inLakeRotorua(VeitchandBell 1990). By 1990, 
bait station technology had been used to eradicate rodents 
from about 18 New Zealand islands, and a similar number 
had been cleared by hand broadcasting baits (Veitch 1994). 
Pacific rats were eradicated from Motuopao Island (30 ha), 
Northland, in 1990, and Motuara Island (59 ha), 
Marlborough Sounds, in 1991, with bait stations placed 

on a 50 m grid (McKenzie 1993; Cash and Gaze 2000). 
To address further the spatial requirement of bait stations 
for Pacific rats, an attempt was made to eradicate Pacific 
rats from Coppermine Island (80 ha) in the Hen and Chick- 
ens group using 100 m spacings. The operation failed but 
it was difficult to determine if this was because of the wider 
bait station spacing or the rugged topography of the is- 
land, which was later cleared of rats using aerial broad- 
cast (McFadden 1997). 

Spacing of bait stations was also a consideration on Allports 
Island (16 ha) in Queen Charlotte Sound in 1989, when 
Derek Brown (1993) successfully employed the Breaksea 
bait station system to eradicate mice using a 50 m grid. In 
exterminating mice from 217 ha Mana Island in 1989 a 
25 m spacing of bait stations was used. However, an aerial 
application was also incorporated into this campaign and 
so a combination of factors brought about success (Hutton 
1990; Hook and Todd 1992). 

We felt throughout our trials that McFadden's concept of 
silo bait-dispensing technology was an alternative or ad- 
junct to ground-based eradications (McFadden and Towns 
1991) and we used "bulk bait stations" in the difficult ar- 
eas at Breaksea. In 1991, the Nelson City Council ap- 
proached us for advice on rats on Haulashore Island (6 ha), 
in Nelson Harbour. Index trapping indicated that this is- 
land had a population of approximately 300 ship rats, which 
we undertook to eradicate. This was a trial of low station 
density, with a weekly checking regime. Two steel 12 gal- 
lon drums placed on their sides were converted into simple 
poison stations by installing a hinged, lockable, access door 
on top, a 100 mm entry hole for rats 25 mm above ground 
level at either end, and a shallow wooden tray for holding 
baits. The drums were positioned about 250 m apart, each 
loaded with 100 "Talon WB 50" baits and checked and 
replenished weekly. In total, 755 baits were taken, about 
2.5 baits per rat, and eradication was achieved over a pe- 
riod of 90 days with minimal operator input. 

This approach worked on a temperate island with a simple 
habitat structure, where rats were scavenging for seasonal 
foods and beach flotsam. However, it is unlikely to be suc- 
cessful in complex habitats (i.e. tropical forests), with an 
abundance of year-round food. The results on Haulashore 
Island strongly suggested that ship rats, like Norway rats, 
were also susceptible to "peer pressure" in following each 
other to food sources, with the last rats extending their 
home ranges to encounter one or other of the two poison 

As a result of several years of trials and investigations into 
how best to deal with the presence of two rat species in a 
single eradication campaign, the Department of Conser- 
vation successfully eradicated Norway and Pacific rats 
from Kapiti Island in 1996 using helicopter broadcast of 
Talon 7-20 pollard baits. Offshore stacks were treated by 
either aerial or hand broadcast of baits and bait stations 
were used on three small adjacent Islands (Empson and 
Miskelly 1999). 


Thomas and Taylor: Ground-based rodent eradication techniques 


With the more economical and simpler aerial poisoning 
operations gaining wider acceptance in New Zealand, op- 
portunities for further testing the bait station approach were 
diminishing. However, ground-based rat eradication tech- 
nology provided a viable alternative in situations where 
aerial broadcast was not feasible or actually prohibited by 
law. This assertion was tested in 1995 when the ground- 
based techniques developed in the Breaksea and Ulva Is- 
land campaigns were extrapolated to much larger Langara 
Island (3100 ha) in the Queen Charlotte Islands, British 
Columbia, Canada. The island had once been one of Brit- 
ish Columbia's largest seabird colonies, with six species 
of burrow-nesting seabirds. Over a period of 30-40 years, 
Norway rats had exterminated five of these as breeding 
species and reduced the others from 200,000 to 14,600 
breeding pairs (Harfenist 1994). Using funds from the liti- 
gation settlement following an oil spill from the tanker 
Nestucca, this project was managed by Environment 
Canada (Kaiser et al. 1997). 

The Langara operation was huge, and new considerations 
such as different habitat type, vulnerable non-target spe- 
cies and the presence of permanent settlements on the is- 
land needed to be catered for (Taylor 1993). The island 
was divided into five working units, each with a camp, a 
supervisor, a cook, and a team of six bait station opera- 
tors. With 100 x 100 m spacings, it required close on 4000 
Breaksea-type bait stations to attain full coverage of the 
island. The whole island was poisoned simultaneously 
using a baiting protocol in which checks and replenish- 
ment of baits were undertaken every two days. Within 
300 m of the shoreline, where concentrations of rats were 
greatest, tunnels were loaded with 12 baits per station. 
Stations over the rest of the island received six baits. Apart 
from a few "stragglers" in two areas of human habitation 
(one was trapped and the others quickly dealt with by pro- 
viding extra baits), eradication of rats from Langara was 
essentially achieved in less than four weeks from the time 
the first bait was laid (Kaiser et al. 1997; Taylor et al. 


The bait station rodent eradication technique is based on a 
strategy that takes into account the characteristics of sec- 
ond-generation anticoagulant poisons, the behaviour of the 
target rodents, and the island environment. It is designed 
to monitor its own progress, kill every rat or mouse within 
a selected timeframe, continually detect the presence of 
surviving rodents, limit the risk to non-target species, and 
overcome many of the problems often associated with "get- 
ting the last rat". This technique has led to many successes 
in rodent eradication, dealing with Norway, ship, and Pa- 
cific rats and house mice on scores of islands, up to 3 1 00 ha 
worldwide. Experience shows that 100 m spacing between 
bait stations is adequate for Norway rats in temperate re- 
gions. In New Zealand, ship rats, Pacific rats, and mice 
have all been eradicated successfully using bait stations 

spaced 50 m apart. Not all operations have been straight- 
forward, but most problems have been associated with 
human populations, non-target species gaining access to 
inadequately designed bait stations, and poison resistance. 
In a few populations where these poisons have been used 
for "controlling" numbers of rats and mice over a long 
period, poison resistance is continuing to be a problem - 
even with second generation anticoagulants - (Quy et al. 

Considering all the campaigns in which we have been in- 
volved, from Awaiti Island (2 ha) in the Marlborough 
Sounds, New Zealand, to Langara Island (3100 ha) in 
Canada, it is evident that the under-pinning factor and key 
to success was the simplicity and self-monitoring nature 
of this ground-based technique. The duration of such eradi- 
cations is related to bait availability in an exponential man- 
ner - the more bait available, the faster the rats are killed 
(Fig. 1). The scaling-up of the technique from Breaksea to 
Langara was straightforward and did not require any sig- 
nificant changes to the methodology. With such a large- 
scale operation, involving a great number of people, many 
inexperienced, there was plenty of potential for problems 
to arise on Langara. However, the success of the campaign 
within its predicted timeframe demonstrates the reliability 
and inherent robustness of the method. 

Ground-based eradication techniques have an important 
part to play in many parts of the world, despite the present 
emphasis in New Zealand on aerial operations (Cromarty 
et al. 2002). In the United States, Canada, and some Euro- 
pean nations, there is legislation regarding the use of ro- 
denticides out-of-doors, and the broadcasting of vertebrate 
poisons from the air is prohibited. Already, there are limi- 
tations on the aerial sowing of anticoagulant rodenticides 
on the New Zealand mainland and on stocked islands. Such 
restrictions are likely to become more widespread in fu- 
ture. Most of the world's biodiversity occurs in tropical 
regions (Africa, Madagascar, Indo-Pacific, South America, 
etc.) where the indigenous inhabitants lack financial re- 
sources to fund aerial operations, whereas labour is less of 
a problem. Bait station techniques are also the only option 
on densely-populated islands. On sparsely-populated is- 
lands, or where very vulnerable non-target species have a 
proscribed distribution, a mix of both methods is often 
appropriate. There is a need for continued development 
of bait station design - in particular, improvements aimed 
at excluding non-target species and limiting the entry of 
toxins into the environment. 

Island rodent extermination campaigns that 20 years ago 
were thought impossible are now being tackled with confi- 
dence - by both ground-based and aerial operations. Two 
important developments made this possible. First, the ad- 
vent of new, potent and highly-palatable "second-genera- 
tion" anticoagulant poisons, and second, the design of ro- 
dent eradication strategies to take advantage of these new 
poisons and our increasing knowledge of rat behaviour. 
However, one of the most important breakthroughs allow- 
ing eradication to progress has been psychological - the 
acceptance that the job can actually be done (Thomas and 


Turning the tide: the eradication of invasive species 

Taylor 1988). It is perhaps ironic that eradication of Nor- 
way rats had been achieved unwittingly from 32 ha Titi 
Island during "rat control" operations that ceased in 1975, 
but through lack of monitoring this remained unknown for 
many years. If this success had been revealed earlier, es- 
pecially at the time of the 1976 symposium, progress in 
the development of rat eradication techniques may have 
been accelerated. This highlights the need for adequate 
monitoring of control and eradication programmes. 

Today, hundreds of rodent eradication campaigns have 
been carried out around the world, and others are in 
progress. Not all are successful. The main reasons for fail- 
ure are that the best poisons are not always used, there is a 
super abundance of alternative foods, there are complica- 
tions with non-target species, the effort is not sufficiently 
organised and sustained, or rodents are able to re-infest 
either from boats or by swimming. Before any campaign 
begins, the chances of re-infestation must be thoroughly 
assessed, and plans formulated and actioned to detect and 
counter future invasions. 


Major conservation advances such as those described in 
this paper are not made alone and we recognise and ac- 
knowledge the huge input and help of persons and 
organisations far too numerous to name here. We thank 
Ian McFadden, David Towns, and Richard Harris for their 
input and comments on the manuscript, and Paddy Sleeman 
and another anonymous referee for useful suggestions now 
incorporated into the text. We also value the help and sup- 
port given by Dick Veitch. 


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